Recently, a personal computer, a television, a VTR, a video disc and the like have become popular and a color display and the like have been widely used as information terminals. The demand for printers for outputting colored still pictures of these terminals has increased. Examples of the recording system of a full-color printer include an electrophotographic system, an ink jet system and a thermo-sensitive transfer system. Of those systems, the thermo-sensitive transfer system has been widely used because it generate little noise and can easily be maintained.
In the thermo-sensitive transfer system, a thermo-sensitive transfer paper in which color ink is fixed and a sheet of image reception paper are used, and recording is made in a manner so that the ink is fusion-transferred or sublimation-transferred onto the image reception paper by controlled thermal energy of a laser, a thermal head or the like in accordance with electric signals.
That is, thermo-sensitive transfer systems may be grouped into a system of the thermal-fusing transfer type and a system of the sublimation-transfer type which use sublimation dyes.
In the system of the heat-fusing transfer type, an ink sheet carrying pigment or dye bound thereon with thermo-fusible wax is used. When the pigment or dye is transferred onto the image reception paper, the wax fused by the thermal energy of the thermal head is also transferred together with the ink. This system of the thermal-fusing transfer type therefore has a disadvantage that it is difficult to obtain half tones required for the necessary image quality and that it is impossible to obtain good hue because of the presence of the transferred wax.
On the other hand, the system of the sublimation-transfer using sublimation dyes is an application of the conventional sublimation-transfer textile printing technique. In this system, a sheet having thereon a dispersed dye which can be relatively easily sublimated bound with a binder is used, so that the dye is sublimated and transferred onto the image reception paper to thereby obtain an image thereon by thermal energy from a thermal head. The sublimation dye is sublimated in accordance with the thermal energy of the thermal head; accordingly, this system has advantages in that it is possible to obtain half tones easily and it is possible to selectively control graduation. The sublimation-transfer system is the most suitable for a full-color printer.
As the thermal transfer image reception paper for use in the thermal transfer system of this sublimation-transfer type, that in which a layer made of thermoplastic resin, which may be effectively dyed by the sublimation dye, such as polyester resins, polyamide resins, epoxy resins, or the like (hereinafter the layer being simply referred to "a dyeable resin layer") is provided on printing base paper which functions as a base material of an image reception paper, as disclosed, e.g., in JP-A-57-107885 (the term "JP-A" as used herein means an unexamined published Japanese patent application).
In the thermal transfer image reception paper provided with such a dyeable resin layer formed on a base material, in the case where ordinary paper is used as the base material, it is necessary to make the voltage to be applied to a thermal head high because the color density is generally low in comparison with synthetic paper, and the color density becomes irregular because of large surface unevenness in ordinary paper. In the case where synthetic paper having a single layer structure, that is, synthetic paper made of polyolefin, polystyrene, or the like, is used as the base material, there is an advantage in that it is possible to obtain sufficient color density as well as excellent image quality. In the latter case of using synthetic paper, however, there are problems. Because synthetic paper is generally stretched so as to increase strength and to provide fine pores, the synthetic paper is shrunk when it is heated in printing by a thermal head only from the side at which the dyeable resin layer is provided. Therefore, distortion is caused between the opposite surfaces of the base material to thereby cause remarkable curl.
In order to prevent these difficulties, i.e., occurrence of curl, then using synthetic paper, there has been proposed an image reception paper base material constituted by two layers of synthetic paper and a backing material (backing layer). That is, a dyeable resin layer is formed on one surface of synthetic paper, and a plastic film or cellulose type fibrous paper is provided as a backing layer on the other surface of the synthetic paper, so that shrinkage of the synthetic paper due to heat upon printing is prevented by the rigidity of the backing layer to thereby prevent the occurrence of curl. The curl amount (.delta.) occurring in the image reception paper base material having such a two-layer structure upon printing may be obtained based on the bimetal theory as follows. ##EQU1## wherein E.sub.1 and E.sub.2 represent Young's moduli of the backing material and the synthetic paper, respectively; .alpha..sub.1 and .alpha..sub.2 represent coefficients of thermal expansion of the backing material and the synthetic paper, respectively; h represents the total thickness of the synthetic paper and the backing material; l represents twice the length of the image reception paper base material in the longitudinal direction; h represents the thickness of the backing layer which is the same as that of the synthetic paper for convenience; T.sub.1 represents the temperature of the backing layer upon printing; and T.sub.2 represents the temperature of the synthetic paper upon printing.
In equation (1), in the case of synthetic paper, when .alpha..sub.2 &lt;0, thermal shrinkage is caused by heat. Generally, the relation T.sub.2 &gt;T.sub.1 is satisfied. In order to reduce the curl (.delta.) in equation (1), it is effective to select synthetic paper having less thermal shrinkage and to select a backing material having a small coefficient of thermal expansion .alpha..sub.1. It is effective in reducing the curl to increase the thickness h. As to the Young's moduli, the second term of the equation (1). ##EQU2## becomes 6E.sub.2 /E.sub.1 when E.sub.1 &gt;&gt;E.sub.2, 6E.sub.1 /E.sub.2 when E.sub.1 &lt;&lt;E.sub.2, and 3/8 (which is the maximum) when E.sub.1 =E.sub.2. In order to reduce the curl, therefore, it is necessary to select a combination of E.sub.1 and E.sub.2 having as large a difference as possible. Because polyolefin type synthetic paper generally includes fine pores inside, it has Young's modulus E.sub.2 of about 10.sup.8 to 10.sup.9 dyne/cm.sup.2 which is smaller than the Young's modulus of other plastic films of 10.sup.9 to 10.sup.10 dyne/cm.sup.2.
Thus, as to the backing material, a so-called high-rigidity material having a great thickness, a high modulus of elasticity, and a low coefficient of thermal expansion is highly effective in preventing curl upon printing.
On the other hand, not only the curl occurring in printing but also the curl occurring before printing becomes a problem. That is, the flatness of the image reception paper deteriorates under various preserving conditions before printing, and therefore the image reception paper cannot be fed well due to the curl thereof when the image reception paper is fed into a printer thereby making printing impossible. Particularly in printers developed recently, in order to simplify the printing operation, an automatic paper-feed system is used and the occurrence of the curl before printing becomes a more serious problem in view of the need for smooth paper feeding. In other words, the prevention of curl before printing is considered to an indispensable condition for carrying out printing, which is therefore more important than prevention of curl after printing.
In order to prevent the curl before printing when image reception paper is returned to the ordinary state (at room temperature and humidity (60.+-.5% RH)) after it has been stored under a predetermined preserving condition for predetermined hours (for example, 72 hours), it is desirable that the base material of the image reception paper has a single layer structure or, in the case of a multi-layer structure, has as symmetrical structure as possible in the direction perpendicular to the layers. That is, curl hardly occurs if transformation is balanced between the opposite sides of the image reception paper base material when the image reception paper is returned from the state under the preserving condition to the ordinary state.
In view of prevention of curl before printing, use of a highly rigid material as a backing material for the purpose of reduction of the curl after printing not only has no effect in reduction of the curl before printing but rather promotes the curl before printing.
For example, when an image reception paper base material of a two-layer structure in which synthetic paper of 60 .mu.m polypropylene (hereinafter simply referred to "PP") and a 75 .mu.m polyethylene terephthalate (hereinafter simply referred to "PET") film including titanium white are bonded to each other through an acrylic resin tackifier (5 to 10 .mu.m) was left at a temperature of 60.degree. C. for 72 hours, the flatness of the image reception paper base material was maintained and ensured under the temperature of 60.degree. C. because of the stress-relaxation function of the tackifier. But, when the image reception paper base material was returned to the ordinary state, curl having a concave surface at the PP synthetic paper side was observed.
On the other hand, if printing is made on image reception paper in which a dyeable resin layer is provided on the PP synthetic paper side of the base material having such a two-layer structure as described above, the curl after printing is extremely small in comparison to the case where the dyeable resin layer is provided on the base material composed of only PP synthetic paper.
In order to suppress the curl before printing, it is effective to use a base material having a multi-layer structure (for example, three layers, five layers, etc.) which is made as symmetrical as possible. In order to suppress the curl after printing, it is desired that layers other than that on the synthetic paper side having a dyeable resin layer formed thereon bring the restricting effect against the thermal transformation due to printing, as disclosed, for example, in JP-A-U-61-188866, and JP-A-61-258793 and JP-A-62-198497 (the term "JP-A-U" used herein means an unexamined published Japanese utility model application). Accordingly, it has been much desired to develop thermal transfer image reception paper which can satisfy both the requirements described above.